How to create registered users?

I want to use this plugin, but when I click “add to favorites” underneath a photo inside a post, it tells me “only registered users allowed.” How do I create a login panel for visitors? And where does it come from? It works fine when I am logged in through the dashboard.

http://www.sooofunny.com/family-photo/

This is just one single post you can visit.

http://ww.wp.xz.cn/plugins/wp-favorite-posts/